1. Field of the Invention
The invention is generally related to desalination of sea water and is specifically directed to a desalination system adapted for recovering minerals from the feedwater.
2. Discussion of the Prior Art
Japan has used electrodialysis (ED) to recover NaCl from seawater to produce edible salt on a large scale for about 40 years. ED plants have been installed to recover NaCl from seawater to use in chlor-alkali plants. The benefit of recovering NaCl from reverse osmosis (RO) concentrate is that the starting salt concentration is twice that of seawater.
Typically, the energy consumption of salt manufactured with seawater reverse osmosis (SWRO) rejects the feed to an ED plant is 80% of that with seawater as the feed (see: Tanaka, Y., Ehara, R., Itoi, S. and Goto, T. “Ion-exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant,” J. Membrane Sci. 222, 71-86, 2003. 2003).
The idea of combining seawater reverse osmosis (RO) and ED to produce freshwater and NaCl is disclosed in U.S. Pat. No. 6,030,535, as well as a zero discharge flow scheme using RO, ED, NF, caustic (NaOH) precipitation of magnesium hydroxide, and evaporation, see for example, U.S. Pat. No. 7,083,730. However neither of these patents addresses gypsum or boron recovery or removal. In addition the silica is not removed in the ED or NF sections, potentially creating scaling on the membranes when operated with the recycle required for zero discharge.
Gypsum scaling is a well known problem in seawater reverse osmosis (RO) plants and especially in zero discharge desalination plants. Sulfate composes approximately 8 wt % and calcium composes 1-1.5 wt % of the total dissolved solids in seawater. When concentrated the sulfate and calcium in the seawater react to form insoluble gypsum.
Concentration occurs in zero discharge or limited discharge plants due to the extraction of desalinated water and recycle of the brine streams without adequate removal of calcium and sulfate. Periodic polarity reversal is used in ED to limit gypsum and calcium carbonate scaling. However, the recycling required for zero discharge can exceed the limits even with polarity reversal.
U.S. Pat. No. 6,030,535 discloses an ED membrane that is not permeable to sulfate to prevent gypsum formation in the ED concentrate stream. However, significant sulfate and calcium is recycled from the ED diluate stream back to the RO unit potentially creating gypsum scaling on the RO membranes. A large portion of the low salt content ED diluate (2 wt % dissolved salt) must be taken as a discharge purge back to the sea to limit the calcium and sulfate concentration in the RO unit brine discharge stream. It would be desirable to fully utilize the ED diluate stream to produce desalinated water since it contains less salt (2 wt %) than the feed seawater (3.5 wt %).
U.S. Pat. No. 7,083,730 discloses partial soda ash softening of the feed seawater to remove most of the calcium to prevent gypsum scaling. However, this requires a significant amount of caustic and soda ash addition and produces a mixed calcium carbonate, magnesium carbonate softener sludge-for disposal. This increases the operating cost of the plant due to the chemical feed cost, sludge disposal cost and loss of byproduct magnesium hydroxide.
The '730 patent provides for valuable magnesium hydroxide production, but due to the presence of sulfate in the magnesium rich brine stream low cost lime or dolomitic lime cannot be used to precipitate the magnesium hydroxide without contaminating it with gypsum. Caustic (NaOH) must be used which is more expensive. Approximately 1.4 tons of NaOH are required to produce 1 ton of Mg(OH)2. Since the market prices for both NaOH and Mg(OH)2 are typically equal it is not profitable to produce Mg(OH)2 using caustic.
Dolomitic lime is used commercially to precipitate Mg(OH)2 from seawater. Approximately 0.85 tons of dolomitic lime (CaO+MgO) is required to produce 1 ton of Mg(OH)2. The typical market price for dolomitic lime is 30-50% of the price of Mg(OH)2 making its recovery using dolomitic lime profitable. Thus it would be economically desirable to develop an enhanced RO and ED based desalination flow scheme that would allow the use of dolomitic lime to precipitate the magnesium from the seawater brine.
None of the known prior art patent addresses the issue of dissolved boron build up in the recycle ED diluate stream to the RO unit. Boron in the form of uncharged boric acid is a component in seawater that typically must be removed at a 90-95% net rejection from the seawater to produce drinking water and agriculture water that meets the World Health Organization guidelines (0.5 ppm B). Since boron is uncharged this is difficult to achieve with RO membranes even on a once through basis.
In the ED and NF units the uncharged boron remains with the diluate and permeate streams and is recycled to the RO unit. This increases its concentration several fold in the feed stream to the RO causing breakthrough into the desalinated product water.
As described in U.S. Pat. Nos. 4,298,442; 5,250,185; and 5,925,255, by raising the pH to 10-10.5 nearly all (>95%) of the uncharged boric acid is converted to monovalent borate and the uncharged silica is converted to monovalent silicate. This allows the borate and silicate to be captured in the ED concentrate stream. This prevents build up in the recycle ED diluate and membrane scaling. However, as explained in U.S. Pat. No. 5,925,255 the calcium and magnesium content must be reduced to very low levels (<<1 ppm) using upstream ion exchange, and bicarbonate must be essentially removed (<<10 ppm) to prevent CaCO3 or Mg(OH)2 scaling. Although U.S. Pat. No. 5,925,255 allows a simple single pass removal of boron and silica with the dissolved salts, it is not economically feasible for seawater desalination due to the large volume of water that would need to be treated and high magnesium and calcium content in seawater (>1500 ppm). If the ED feedwater pH were raised to 10 a portion of the bicarbonate in the seawater brine would be converted to carbonate and could form a carbonate scale with the calcium dissolved in the ED feedwater. In addition magnesium in the seawater could form magnesium hydroxide and foul the membranes.
U.S. Pat. No. 7,083,730 discloses the use of partial soda ash softening to remove some of the bicarbonate and calcium. However, significant calcium and bicarbonate remain in the feed seawater (typically >50 ppm bicarbonate and >100 ppm calcium) along with most of the magnesium. This gets concentrated in the RO brine and still could form calcium carbonate or magnesium hydroxide precipitate in the ED unit at pH 10. It would be desirable to develop an enhanced RO and ED based desalination flow scheme that would allow the ED to operate at a pH of 10 allowing boron and silica rejection without calcium carbonate or magnesium hydroxide scale formation.
In U.S. Pat. No. 7,083,730 boron contamination of the magnesium rich brine is also a problem for magnesium hydroxide production. The presence of boron is undesirable since it is adsorbed onto the surface of the of the magnesium hydroxide particles, making the magnesium hydroxide unsuitable for refractory brick applications. Additional treatment of the brine (ion exchange, excess lime addition) is required if significant boron is present. It would be desirable to develop an enhanced RO and ED based desalination flow scheme that would produce magnesium rich brine that has a low boron content allowing high purity magnesium hydroxide production without additional treatment steps.
Published US Application, Pub. No. 2008/0237123, discloses a method of regenerating N-Methyl-D-glu-camine-functional resin that has been used for boron-removal uses a closed recirculating loop for treating the conjugate acid salt of the N-Methyl-D-glucamine functionality of the resin. The method reduces rinse water demand and improves pH control in a water treatment system, and can be used to improve the performance of boron-selective resins in stand-alone systems or as a second stage in a reverse osmosis seawater desalination system.
An article by Ki-Won Baek, et al, entitled: “Adsorption Kinetics of Boron by Anion Exchange Resein in Packed Column Bed,” J. Ind. Eng. Chem, Vol. 13, No. 3 (007) 452-456, Discusses an application for Amberlite IRA 743 resin that has glucamine functional groups for specific boron exchange. The boron removal efficiency from seawater was examined through the packed column experiment. The efficiency of boron removal was investigated with respect to several parameters, including the pH of the solution, the initial concentration of boron, the bed volume of the resin, and the temperature. The performance of boron removal increased upon increasing the batch ratio of resin to boron and decreasing the initial concentration of boron in solution. The removal rate of boron is independent of temperature. Most of the boron in aqueous solution could be removed efficiently under the optimum operation conditions at pH 8.5. In addition, a kinetic study of boron removal under the optimum conditions fit Thomas's adsorption model well.
Many of the seawater RO plants currently in operation discharge the entire reject brine stream with no minerals recovery. It would be desirable to retrofit these plants with an ED based system that allows economic recovery of the minerals. However, the prior art systems described here recycle all or a portion of the ED diluate stream back to the RO unit. This recycle increases the flow and changes the composition of the RO feedwater. This requires modifications to an existing RO based desalination facility requiring an extended outage. In addition the change in feedwater composition may cause unforeseen scaling problems in the existing RO unit.
It would be desirable to develop an enhanced RO and ED based desalination flow scheme that could process the brine stream from an existing RO desalination plant without impacting the operation of the existing RO desalination plant.